The mast cells in rat peritoneal cavities have heterogeneous characteristics. They show variation in maturity as well as in their biochemical activities. Mast cells in all 4 stages of development were present in the cavity. Young cells (stages 1 and 2) contained little histamine but had high histamine synthetic activity. They were unable to retain most of their newly synthesized histamine. Mature cells (stages 3 and 4), on the other hand, contained more histamine and had lower histamine synthetic activity than did immature cells. The young cells were resistant to degranulating agents while the mature cells were fully responsive. In cultured rat basophilic leukemia (2H3) cells were similar to immature peritoneal mast cells in having high histamine synthetic activity, low histamine content and high rates of histamine turnover. Changes related to cell maturation were also evident in 2H3 cells. Histamine synthetic activity, histamine content and responsiveness to stimulants increased as cells approach 'S' phase of the cell cycle and then decreased as cells approached mitosis.